[PATCH v3 3/4] unpack-trees: reduce malloc in cache-tree walk

[Nguyễn Thái Ngọc Duy]

This is a micro optimization that probably only shines on repos with
deep directory structure. Instead of allocating and freeing a new
cache_entry in every iteration, we reuse the last one and only update
the parts that are new each iteration.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclo...@gmail.com>
Signed-off-by: Junio C Hamano <gits...@pobox.com>
---
 unpack-trees.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/unpack-trees.c b/unpack-trees.c
index ba3d2e947e..c8defc2015 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -694,6 +694,8 @@ static int traverse_by_cache_tree(int pos, int nr_entries, 
int nr_names,
 {
        struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        struct unpack_trees_options *o = info->data;
+       struct cache_entry *tree_ce = NULL;
+       int ce_len = 0;
        int i, d;
 
        if (!o->merge)
@@ -708,30 +710,39 @@ static int traverse_by_cache_tree(int pos, int 
nr_entries, int nr_names,
         * in the first place.
         */
        for (i = 0; i < nr_entries; i++) {
-               struct cache_entry *tree_ce;
-               int len, rc;
+               int new_ce_len, len, rc;
 
                src[0] = o->src_index->cache[pos + i];
 
                len = ce_namelen(src[0]);
-               tree_ce = xcalloc(1, cache_entry_size(len));
+               new_ce_len = cache_entry_size(len);
+
+               if (new_ce_len > ce_len) {
+                       new_ce_len <<= 1;
+                       tree_ce = xrealloc(tree_ce, new_ce_len);
+                       memset(tree_ce, 0, new_ce_len);
+                       ce_len = new_ce_len;
+
+                       tree_ce->ce_flags = create_ce_flags(0);
+
+                       for (d = 1; d <= nr_names; d++)
+                               src[d] = tree_ce;
+               }
 
                tree_ce->ce_mode = src[0]->ce_mode;
-               tree_ce->ce_flags = create_ce_flags(0);
                tree_ce->ce_namelen = len;
                oidcpy(&tree_ce->oid, &src[0]->oid);
                memcpy(tree_ce->name, src[0]->name, len + 1);
 
-               for (d = 1; d <= nr_names; d++)
-                       src[d] = tree_ce;
-
                rc = call_unpack_fn((const struct cache_entry * const *)src, o);
-               free(tree_ce);
-               if (rc < 0)
+               if (rc < 0) {
+                       free(tree_ce);
                        return rc;
+               }
 
                mark_ce_used(src[0], o);
        }
+       free(tree_ce);
        if (o->debug_unpack)
                printf("Unpacked %d entries from %s to %s using cache-tree\n",
                       nr_entries,
-- 
2.18.0.656.gda699b98b3